Hitherto, a heat-sensitive recording material which utilizes a coloring reaction of a color former and a color developer which forms a color upon contact with the color former and provides a color image by contacting both coloring materials by the action of heat is well known.
Such heat-sensitive recording materials are comparatively inexpensive and can be used on a recording equipment that is compact and requires fairly easy maintenance. Because of these advantages, heat-sensitive recording materials are extensively used not only as recording media in facsimile equipments and various types of computers but also in a wide range of applications including heat-sensitive labels. One major problem with heat-sensitive recording materials is, however, their low resistance to fingerprints or solvents; if the recording layer comes in contact with a sebum of human being or a solvent, the image density of recorded characters is decreased or an unwanted coloration ("background fog") occurs.
With a view to solving this problem, several methods have been proposed; in one method, a heat-sensitive recording layer is coated with an aqueous emulsion of a resin having film-forming properties and resistance to chemicals (Japanese Patent Application (OPI) No. 128347/79) ; and in another method, a heat-sensitive recording layer is coated with a water-soluble high molecular weight compound such as polyvinyl alcohol (Japanese Utility Model Application (OPI) No. 125354/81). (The term "OPI" as used herein means a "published unexamined application".) However, the methods so far proposed either suffer from additional problems or find themselves incapable of achieving the intended results to satisfactory levels. For instance, in the method in which an aqueous resin coating is applied to the heat-sensitive recording layer, the temperature for drying has to be limited to a certain level in order to avoid unwanted coloration of the recording layer due to high-temperature drying, whereby curing of the resin layer inevitably becomes insufficient for preventing its sticking to a recording head during the recording. In order to avoid this problem, a method in which a heat-sensitive recording layer is coated with a resin component capable of curing upon exposure to electron beams and the coated resin component is cured upon exposure to electron beams is proposed. However, the resulting heat-sensitive recording material is still unsatisfactory in terms of preservability of recorded images. In addition, there may also be problems that the electron beam-curable resin layer causes coloration of the heat-sensitive recording layer just after it has been coated or causes fading of recorded images.
In order to solve these problems, the inventors made extensive investigations. As the result, they found that when an interlayer of an aqueous resin is formed on a heat-sensitive recording layer and an overcoat layer containing a resin that is curable upon exposure to electron beams is formed on the interlayer, there is obtained a heat-sensitive recording material which exhibits not only superior recording characteristics as well as widely varying surface characteristics but also good preservability of recorded images without being attended by fogging of the recording layer and, thus, formerly filed Japanese Patent Application No. 124562/86 (corresponding to U.S. application Ser. No. 51,599, filed May 20, 1987).
On the other hand, recently, use has been made of various kinds of printers such as video printers, which can provide images of high qualities comparable to photographs, and even with regard to a heat-sensitive recording material for printing out, it is requested to develop a recording material which is excellent in density as well as gradation of recorded images. For this purpose, heat-sensitive recording materials using a plastic film or a synthetic paper as a support are being developed. But, even in such heat-sensitive recording materials, in order to prevent fading of recorded images, an overcoating such as aqueous resins is applied onto the recording layer.
However, when an overcoat layer is provided on a heat-sensitive recording material using a plastic film or a synthetic paper as a support, it was brought to light that the overcoat layer shows a tendency to stick to a recording head or a paper delivery guide and eventually causes paper jam especially in the case where the recording is carried out under conditions of high humidity. The density of recorded images is also still unsatisfactory.
In view of these circumstances, the inventors further made elaborate investigations to develop a heat-sensitive recording material comparable to a photograph, which is not only especially high in density of recorded images but also excellent in recording characteritics such as gradation. As the result, they have found that when an interlayer having a specified surface smoothness is provided on a recording layer of a heat-sensitive recording material using a plastic film or a synthetic paper as a support and an overcoat layer containing a resin that is curable upon exposure to electron beams is then provided on the interlayer, there is obtainable a heat-sensitive recording material which is not only high in density of recorded images and excellent in gradation and preservability but also is characterized by that it has widely varying surface characteristics as compared with those in the case of ordinary paper being used as a support and shows no sticking to a recording head or the like even when the recording is carried out under conditions of high humidity.